Per The Hill, 43 percent of respondents said they would vote for the Democratic Party's presumptive presidential nominee if the election were held today, while 39 percent said they would back Trump.
Additionally, 5 percent of voters said they would not vote for either of the candidates, while another 5 percent said that they will not vote at all in the 2020 presidential election. Only 8 percent of respondents said they were still undecided.
In June's poll, Biden had a 10-point advantage over the president.
According to Democratic pollster Terrance Woodbury, the apparent lack of enthusiasm among Democrats could be a major issue for Biden going forward.
"I think a four point lead should be very concerning. We know the enthusiasm gaps between Biden and Trump, with Trump's supporters being very enthusiastic and insistent on voting for him, margins that close are problematic."As The Hill noted, polling suggests that Trump supporters are enthusiastic about backing their candidate. For instance, a recent New York Times/Siena College poll found large gaps in enthusiasm between the two men in battleground states.
Similarly, a USA Today/Suffolk poll established that only 27 percent of Biden supporters are enthusiastic about voting for him, while around half of Trump supporters are "very excited" about casting a ballot in November.
However, The Hill/Harris-X poll could be an outlier. Other recent polling suggests that the former vice president is still comfortably ahead.
A poll from Rasmussen Reports released on Wednesday put Trump 10 percentage points behind Biden nationwide.
The poll showed that Trump is supported by 40 percent of likely voters, while 50 percent of them back Biden. Only 6 percent said they were undecided.
The survey also found that Trump enjoys the support of 74 percent of Republicans, while 79 percent of Democrats support Biden. Among those unaffiliated with either of the major parties, Biden polled at 48 percent and Trump at 36 percent.
Some Republicans have expressed concern over Trump's political standing. Notably, former President George W. Bush's deputy chief of staff, Karl Rove, recently called on the commander-in-chief to "restart" his re-election campaign.
"When you are in the barrel. When you're getting a lot of bad press and the polls are going against you, you need to do something that says, 'We're moving in a different direction,'" Rove told Fox News.
Rove also said that Trump needs to "have a bold agenda that he lays out for a second term and that he can contrast with Joe Biden."